[ASTImporter] Implement some expression-related AST node import.

Introduce ASTImporter unit test framework.

Fix a memory leak introduced in cf8ccff5: an array is allocated
in ImportArray and never freed.

Support new node kinds:

- GCCAsmStmt

- AddrLabelExpr
- AtomicExpr
- CompoundLiteralExpr
- CXXBoolLiteralExpr
- CXXNullPtrLiteralExpr
- CXXThisExpr
- DesignatedInitExpr
- GNUNullExpr
- ImplicitValueInitExpr
- InitListExpr
- OpaqueValueExpr
- PredefinedExpr
- ParenListExpr
- StmtExpr
- VAArgExpr

- BinaryConditionalOperator
- ConditionalOperator

- FloatingLiteral
- StringLiteral

- InjectedClassNameType
- TemplateTypeParmType

- LabelDecl

Patch by Aleksei Sidorin!

Differential Revision: http://reviews.llvm.org/D14286

llvm-svn: 266292